Date: Wed, 21 Jan 2009 14:07:43 -0500 From: John Baldwin <jhb@freebsd.org> To: freebsd-hackers@freebsd.org Cc: Andriy Gapon <avg@icyb.net.ua> Subject: Re: usb keyboard vs btx: an SMI theory Message-ID: <200901211407.44021.jhb@freebsd.org> In-Reply-To: <4947AA3C.4040005@icyb.net.ua> References: <4947AA3C.4040005@icyb.net.ua>
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On Tuesday 16 December 2008 8:16:44 am Andriy Gapon wrote: > Again, I am very fuzzy about the exact details, but I think that this is > something that could be happening and I think that SMI is of primary > interest here. I also think that this might explain to a certain degree > the difference in behavior between "older" btx and "newer" btx. One thing to keep in mind is that when an SMI# is delivered, the processor enters System Management Mode (SMM). In SMM, the CPU actually uses a different set of memory for its RAM. It also runs in a sort of weird 32-bit real mode. It is not going to call the stock IRQ 1 handler. Instead, it passes data back to "normal" mode by changing the values restored into registers when exiting SMM. Typically doing an I/O port access to the ports backing the keyboard (0x60 and 0x64) cause an SMI# and the SMM handler emulates the inb/outb request by storing the resulting data for an inb in the %ax register the "normal" mode sees once it resumes execution after the 'inb' instruction. -- John Baldwin
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